1. Field of the Invention
The present invention relates, in a vibration actuator represented by an ultrasonic motor, to a vibration actuator in which two vibrational modes are used, for example, a longitudinal vibration and a torsional vibration. In particular, the present invention relates to a vibration actuator having an improved construction and to a method of forming same.
2. Description of the Related Art
FIG. 13 is an oblique view showing a conventional vibration actuator of the type using a longitudinal vibration and a torsional vibration.
Heretofore, in this kind of vibration actuator, the stator (fixed element) 101 comprises a piezoelectric element 104 for torsional vibration, disposed between two columnar type vibration elements 102, 103. Moreover, a piezoelectric element 105 for longitudinal vibration is arranged at the upper side of the vibration element 103. The piezoelectric element 104 for torsional vibration is polarized in the circumferential direction, while the piezoelectric element 105 for longitudinal vibration is polarized in the thickness direction. Furthermore, a rotor (moving element) 106 is disposed on the upper side of the piezoelectric element 105 for longitudinal vibration.
The vibration elements 102, 103 and piezoelectric elements 104, 104 which constitute the stator 101 are fixed by means of threaded retention on the threaded portion of a shaft 107. The rotor 106 is rotatably disposed on the shaft 107 via ball bearings 108. A nut 110 is screw retained on the end of the shaft 107 via a spring 109, causing the rotor 106 to be in compressive contact with the stator 101 with a compressive force F.
The piezoelectric element 104 for torsional vibration and the piezoelectric element 105 for longitudinal vibration are driven by a voltage, phase controlled by means of a phase shifter 112, oscillating at the same frequency, as that supplied from the oscillator 111.
The piezoelectric element 104 for torsional vibration incurs a mechanical deformation in order to rotate the rotor 106. On the other hand, the piezoelectric element 105 for longitudinal vibration, via a frictional force acting between the stator 101 and the rotor 106, plays a clutch-like role in converting vibration to unidirectional motion, by means of causing periodic vibration synchronously with the period of the torsional vibration due to the piezoelectric element 104.
FIG. 14 is an oblique exploded view of the stator of the conventional vibration actuator.
It is necessary to polarize in the circumferential direction the piezoelectric element 104 for torsional vibration. Because of this, the piezoelectric material, as shown in FIG. 14, once divided into about six to eight fan-shaped pieces, and after each piece had been polarized, they were reassembled into a ring. Moreover, element 104a is an electrode.
However, in the conventional vibration actuator, when assembling the piezoelectric element 104 for torsional vibration into a ring, it was difficult to provide shape accuracy.
On the other hand, the areas of the piezoelectric element 105 for longitudinal vibration and the piezoelectric element 104 for torsional vibration, were both about equal to the cross sectional area of the stator 106, or, were smaller than the cross sectional area of the stator 106. Moreover, it was necessary to open a hole in the respective centers of the piezoelectric element 105 for longitudinal vibration and the piezoelectric element 104 for torsional vibration, in order for the shaft 107 to pass therethrough. Because of this, the respective areas of the piezoelectric element 105 for longitudinal vibration and the piezoelectric element 104 for torsional vibration became further reduced, and it became difficult to attain both high torque and high rotation of a vibration actuator.
In order to solve such problems, the present assignee has already disclosed, in Japanese Patent Application 6-275022, a type of vibration actuator which uses longitudinal vibrations and torsional vibrations and which can drive at high torque and high rotation, and which moreover is of simple structure and simple to manufacture.
FIG. 15 is a cross sectional diagram showing the structure of this vibration actuator. In FIG. 15, a cylindrical elastic member 202 is arranged in the outer circumferential surface of a rod-shaped fixed shaft 201 which has a large diameter portion 201a in the center. The elastic member 202 is threadingly held in the large diameter portion 201a by means of mounting bolts 203a, 203b.
The elastic member 202 is constituted by bringing about the assembly of two thick walled semicylindrical elastic members 202a, 202b. In the junction surface between the elastic bodies, two piezoelectric elements for torsional vibration using a piezoelectric constant d.sub.15 and two piezoelectric elements for longitudinal vibration using a piezoelectric constant d.sub.31 (both not shown in the drawing) are superposed. Thus a total of four piezoelectric elements are present.
A moving member (relative moving member) 205 is in contact with the driving surface D which is the upper end face of the elastic body 202. This moving member 205 is disposed to rotate freely on the fixed shaft 201 by means of a bearing 204 arranged in the center.
The moving member 205 is constituted by a moving member base material 205a and a sliding member 205b which is in contact with the drive surface D of the elastic member 202. By means of the bearing 204 which has been fitted into its inner circumference, the moving member 205 is then located with respect to the fixed shaft 201.
Moreover, the moving member 205 is in contact, in a compressed state, with the drive surface D of the elastic member 202, due to a compression member 206 which is a disc spring, a coil spring, or a plate spring. By this means, while driving as a vibration actuator, it is such that shaft vibration is not generated. This fixed shaft 201 has a threaded portion 201b formed at the front end thereof. An adjustment member 207, which is a nut or the like to regulate the amount of compression of the compression member 206, is held by means of screw threading onto the other end of fixed shaft 201.
A vibration actuator which has been constituted in this manner is excited by means of an impressed drive voltage from drive voltage elements (not shown in the drawing) or a drive voltage source (not shown in the drawing). Torsional vibrations and longitudinal vibrations are generated in the elastic member 202. When the resonant frequencies of the torsional vibration and the longitudinal vibration are about in agreement, torsional vibrations and longitudinal vibrations arise synchronously (that is, degeneracy), and elliptic motion arises in the drive surface D. This elliptic motion consists of a drive force, and the moving element 205 which is in compressive contact revolves around the fixed shaft 201.
However, the size of the locus of the elliptic motion which arises in the drive surface D of the elastic member 202 is about 1-2 .mu.m. Because of this, for the elliptic motion generated in the drive surface D of the elastic member 202, in order to drive the moving element 205, to reliably reach the moving element 205 with which member 202 makes compressive contact, it is necessary to control to a predetermined surface roughness the drive surface D of the elastic member 202. This is done by, for example, performing lapping treatment or the like polishing or grinding treatment.
However, as shown in FIG. 15, the elastic member 202 is constituted as two assembled semicylindrical elastic members 202a, 202b. Because of this, the polishing or grinding treatment of the drive surface D, in order to obtain a predetermined surface roughness, is necessarily performed after the assembly of the elastic member 202.
Namely, in order to perform the polishing or grinding treatment of the drive surface D of the elastic member 202 of the vibration actuator shown in FIG. 15, the following steps from (1) through (6) are required.
(1) Four piezoelectric elements are interposed between elastic members, and in addition, in a state with adhesive caused to be interposed, the semicylindrical elastic members 202a, 202b are arranged opposite one another in the periphery of the fixed shaft 201. PA0 (2) The semicylindrical elastic members 202a, 202b are fixed by mounting bolts 203a, 203b. The semicylindrical elastic members 202a, 202b and the piezoelectric elements are then adhered together. PA0 (3) After adhesion, the mounting bolts 203a, 203b are removed, the elastic member 202 is pulled out from the fixed shaft 201. PA0 (4) Polishing or grinding treatment of the drive surface D of the elastic member 202 is performed. PA0 (5) After the polishing or grinding has ended, the elastic member 202 is mounted on the fixed shaft 201. It is again fixed by the mounting bolts 203a, 203b. PA0 (6) The moving member 205 and the like are mounted on the fixed shaft 201.
Moreover, there is a risk that the adhesive, in order to adhere the piezoelectric elements and the semicylindrical elastic members 202a, 202b, could get around the mounting bolts 203a, 203b. In this case, in the process step (3), the mounting bolts 203a, 203b cannot be removed. To prevent this, for example, it is also necessary for the threads of the mounting bolts 203a, 203b to be coated with silicone oil or a similar adhesion prevention measure, and the process becomes complicated.
In this manner, in the vibration actuator proposed by Japanese Patent Application 6-275022, a large number of process steps are required, including grinding or polishing the drive surface D of the elastic member 202, and there is a risk of the process impeding mass production.
Furthermore, in the aforementioned method, when reassembling the support member 201 and the elastic body after polishing or grinding of the drive surface D, it is difficult to set the angle between the support member 201 and the drive surface D correctly at 90.degree.. Because of this, a skew arises between the drive surface D and the moving member 205, the drive force or driving effect is reduced, and the occurrence of noise and similar problems arise.